Triacylglycerols (TAG) are the principal carbon storage compounds in various organisms, including vertebrates, oilseed plants, oleaginous fungi, yeast and microalgae. In microalgae, TAGs are mainly accumulated in extraplastidial oil bodies, and are implied as potential resource of neutral lipids for biodiesel production.
TAG is synthesized de-novo by a sequential transfer of fatty acyl chains from acyl-CoA through the glycerol-3-phosphate (G3P) pathway, also known as the Kennedy pathway. Briefly, G3P is first acylated by the action of the acyl-CoA: glycerol-3-phosphate acyltransferase (GPAT), followed by a second acylation step catalyzed by the acyl-CoA: lysophosphatidate acyltransferase (LPAAT). The phosphatidic acid obtained is then dephosphorylated by a phosphatidate phosphatase (PAP) to generate diacylglycerol (DAG), which is finally used as substrate for the acyl-CoA: diacylglycerol acyltransferase (DGAT) to produce TAG. Consequently, DGAT catalyzes the final and committed step for TAG biosynthesis.
Three types of DGAT enzymes commonly referred to as DGAT type 1, 2, and 3, may take part in the acyl-CoA-dependent formation of TAG. Two major isoforms are encoded by DGAT1 and DGAT2 genes that have been identified to encode for distinct proteins which both function as DGAT enzymes and are responsible for the bulk of TAG synthesis in most organisms. Cytosolic DGAT3 has been discovered in peanut Arachis hypogea, but seems to play only a minor role in plant TAG formation.
DGAT1 (EC 2.3.1.20) proteins are members of the MBOAT (membrane-bound O-acyltransferase) protein superfamily and differ structurally from DGAT2 proteins. DGAT1 proteins are larger than DGAT2 and possess at least six transmembrane domains compared to the two predicted in DGAT2 (Yen et al., 2008). The emerging role of DGAT2 orthologs seems to be more important for incorporation of unusual fatty acids in the seed storage oils of some plants. In addition, the bifunctional DGAT/wax ester synthase from Acinetobacter calcoaceticus, is also a member of the MBOAT superfamily, however, for its higher plants homologs, the DGAT activity is either absent or much lower relative to the wax synthase activity.